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Garnet Group ( X 3 Y 2 Z 3 O 12 )

Garnet Group ( X 3 Y 2 Z 3 O 12 ). Garnets are diverse compositionally, physically, and in mode of occurrence -        the garnet structure accommodates cations with a wide range of sizes and valence states   - cation ordering lowers symmetry

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Garnet Group ( X 3 Y 2 Z 3 O 12 )

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  1. Garnet Group(X3Y2Z3O12)

  2. Garnets are diverse compositionally, physically, and in mode of occurrence -        the garnet structure accommodates cations with a wide range of sizes and valence states   - cation ordering lowers symmetry (Takeuchi and Haga, 1976) The color ranges from white through shades of Red, brown, yellow and green, to black Stable phases in a wide range of pressures, temperatures, and chemical environments

  3. Garnet Structure Crystal system = Isometric Point group/crystal class = 4/m –3 2/m Space group = Ia 3d a=11.459-12.058 Z=8

  4. Garnet Structure Site X Y Z C. N. 8 6 4 Fractional Coordinate x 1/8 0 3/8 y 0 0 0 z 1/4 0 1/4

  5. Garnet End-Members(Winchell, 1933) • Ugrandite GroupPyralspite Group • Uvarovite Ca3Cr2(SiO4)3 Pyrope Mg3Al2(SiO4)3 • Grossular Ca3Al2(SiO4)3 Almandine Fe3Al2(SiO4)3 • Andradite Ca3Al2(SiO4)3 Spessartine Mn3Al2(SiO4)3

  6. Garnet Occurrences Pyrope - mineral of the upper mantle - ultra-basic rocks Almandine - common garnet in metamorphic rocks - regional metamorphism of argillaceous sediments Spessartine - occurs in skarn deposits and Mn-rich deposits Grossular - contact or regional metamorphism of impure limestones Andradite - may be the result of metamorphism of impure siliceous environments similar to that of grossular Uvarovite - rarest of garnets - found in serpentine associated with chromite

  7. Compression of Garnet at 300K End Member Pyrope Grossular Andradite Pmax (Gpa) 5.6 6.1 19.0 Site bulk moduli (GPa) x 130(10) 115(13) 160 y(Al) 220(50) 220(50) 330(33) z(Si) 300(100) 300(100) 200(20) Reference Hazen & Finger Hazen & Finger Hazen & Finger (1989) (1978) (1989) -

  8. Garnets are differentiated by slight variations in color, density, and index of refraction. -         - They have high indices of refraction -         - Range of beautiful colors -         - Transparent -         - Lack cleavage and are durable Thus, make good candidates for gemstones -         - semi-precious -         - abundant - not as hard as other gemstones

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